This invention relates to rotary hammers, and, in particular to hammers that incorporate an air cushion hammering mechanism.
Such hammers will normally include a tool holder that can hold a hammer bit or chisel bit for acting on a workpiece, and an air cushion hammering mechanism which comprises a piston and a beat piece that are slidably located in a cylinder so that reciprocation of the piston in the cylinder will cause the beat piece to hit a bit located in the tool holder. Such hammers may, however, be employed in more than one mode. For example a hammer may be capable of being employed in a hammer only or so-called "chiselling" mode in which the piston reciprocates within the cylinder in order to cause the beat piece to hit the bit without any rotation of the tool, or alternatively a drilling only mode in which the cylinder may form part of a spindle connected to the tool holder and is caused to rotate about the piston, thereby causing the bit inserted in the tool holder to rotate. The hammer may also be capable of being employed in a combination rotary hammer mode in which the piston reciprocates within the cylinder causing the beat piece to hit the bit while at the same time the cylinder rotates about the axis of the piston, thereby causing the bit to rotate.
As an example, one such hammer is described in WO 98/47670. This hammer has a drive motor that is arranged with its armature shaft at right angles to the axis of the hammer spindle, and has a single switching mechanism that can switch the hammer between pure rotation, pure chiselling and combination rotation and chiselling modes. The armature shaft of an electric motor is coupled to a drive shaft on which is mounted one end of crank arm that causes the piston to reciprocate within a horizontally oriented cylinder when the drive shaft rotates. The piston is linked to a ram also located in the cylinder by means of an air gap so that reciprocation of the piston causes the ram to reciprocate and to hit a beat piece located forward of the ram, thereby causing the beat piece to impact the rear end of the bit that is inserted in the tool holder. The mode of operation may be changed by means of a switch into a rotary mode in which the piston crank is decoupled from the drive shaft, and instead the cylinder is caused to rotate about the piston, ram and beat piece, thereby causing the bit to rotate in the tool holder. By moving the switch to a third position, the piston can be caused to reciprocate while the cylinder rotates, thereby putting the bit into rotary hammering or chiselling mode.
The various reciprocating parts of the hammer, and in particular the ram, must be sealed within the cylinder against pressure differences occurring on different sides thereof while at the same time being able to move within the cylinder. This has, however, been the cause of a number of problems: Generally such seals have been formed as an annulus of an elastomeric material located within a groove, which annulus sits slightly proud of the surface of the ram in order to seal the ram within the cylinder. In addition, the seal and/or the inner surface of the cylinder must be provided with a coating of grease in order to maintain the seal. However, reciprocation of the ram within the cylinder will gradually wipe the grease away from the region of the seal and so reduce the sealing effect of the seal and increase frictional heating of the seal in the cylinder due to the reciprocation of the ram.